1. Field of the Invention
This invention concerns a seal mechanism for a ball valve.
2. Description of the Prior Art
Seal members for use in ball valves have been known to include elastic seats made of rubber or the like having a relatively soft nature and a plastic seat or metal seat having a relatively high degree hardness. Since the latter rigid seats are lacking in sealing performance under cold temperature conditions such as at several tens of degrees Centigrade below the freezing point, it is desirable to use the former elastic seat under such a condition.
However, since the elastic seat is soft in nature, a problem that occurs is that the portion of the seat that initially faces the through-hole of a ball-like valve body upon the initial stage of valve opening operation will bulge out of a seat retaining groove.
A counter-measure for resolving such a problem is proposed in Japanese Patent Laid-Open No. 63061/1980, which is illustrated in FIG. 1 of this application.
In a ball valve illustrated in FIG. 1, a half-groove is formed along the outer periphery at the top end of an inner ring 3a and a half-groove is formed along the outer periphery at the top end of an outer ring 3b, and the half-grooves constitute a seat retaining groove 3c. A valve seat is 4 made of rubber is tightly forced into the groove 3c and adapted such that the radial dimension d1 for a sealing point S1 at the corner 4a on the top end along the inner circumferential surface of the valve seat in pressure contact with a ball-like valve body 5 while a valve seat 4 also bulges out of the seat retaining groove 3c. Radial dimension d1 is made larger than the radial dimension d2 for a sealing point S2 at the corner 4b on the end opposite to the top end (hereinafter referred to as a counter top end) along the inner circumferential surface of the valve seat that faces the side wall 3d on the counter-top end of the inner ring 3a that forms the above mentioned half-groove.
According to the foregoing structure, as illustrated in FIG. 2, since the inner circumferential surface 4c of the seat 4 is inclined, the fluid pressure P1 acting vertically on the inner circumferential surface 4c is exerted as a component P1(Y) directed radially outwardly and as a component P1(X) directed away from the valve body with respect to the fluid passing direction, whereby the seat 4 is continuously forced into the groove 3c and, accordingly, the seat can effectively be prevented from bulging out of the groove 3c.
However, the seal mechanism constituted as described above having the seat 4 made of rubber with hardness of about 80 presents a problem in that the effective prevention for the bulge of the seat 4 can no more be ensured if the fluid pressure P1 increases higher than about 120 kg/cm.sup.2.
In view of the above, the present inventors have studied the foregoing problems and found that the bulging phenomenon of the seat as described above is caused by the following manner.
Specifically, when the valve body 5 is revolved in the direction of an arrow R as shown in FIG. 3(A) and a portion of the seat 4 is begins to separate from the circumferential surface 4c of the seat 4, the corner 4a of the seat is bulged to the outside of the groove since a high pressure, e.g., of greater than 120 kg/cm.sup.2 is applied to the inner circumferential surface 4c of the seat 4. Then, accompanying the bulge or the protrusion of the corner 4a, the sealing point S1 is displaced radially inwardly. On the contrary, since the volume of the seat 4 is constant, the corner 4b on the counter top end displaced radially outwardly by the amount corresponding to the bulge of the corner 4a. Then, when the valve body 5 is further opened as shown in FIG. 3B, the above-mentioned phenomenon becomes more conspicuous, until the diameter for the sealing point S2 becomes greater than the diameter for the sealing point S1, whereupon the fluid pressure acts in the direction of enforcing the seat 4 out of the groove 3c. Thus, when the seat 4 completely detaches from the valve body 5 and opposes to the through hole 6, the relevant portion of the seat 4 instantaneously bulges out of the retaining groove 3c.
As described above, the bulging phenomenon of the seat is caused, upon valve opening action taken under the state where a high fluid pressure is being applied to the inner circumferential surface of the seat, by the change that the radial size for the sealing point S1 of the seat 4 relative to the valve body is decreased as compared with the radial size for the sealing point S2 of the seat relative to the side wall on the counter-top end of the inner ring.